Bleaching of wood pulp with thiourea dioxide and zinc sulfate



Unitcd States Patent Int. cl. D21c 3/20 vs. Cl. 162-42 3 Claims ABSTRACTOF THE DISCLOSURE The invention consists in a method of bleaching woodpulp, in which a bleaching composition is used containing 0.2% to 1.0%of thiourea dioxide and 1% to 5% of zinc sulphate, based on the weightof dry wood substance in the pulpto be bleached.

This invention relates to the bleaching of wood pulp. It is particularlyconcerned with the bleaching of mechanical wood pulp, in which theefiiciency of conventional bleaching agents, such assodium'hydrosulphite and sulphoxylate is impaired due to the fact thatsuch substances decompose at an early stage during the bleachingprocess. The invention is however also applicable of chemical wood pulp.

to the bleaching If +30 "According to the invention, wood pulp 1sbleached by.

means of a bleaching composition. which contains thiourea dioxide as theactive bleaching-agent.

When thiourea dioxide is used as the active bleaching agent, there isgreatly increased efficiency per unit quantity as compared withconventional materials, and also the bleaching effect obtained issuperior; brightness in creases of from 5 to 10 points as measured bythe calcium block brightness test can be achieved-and the brightness hasgreater permanency.

The effectiveness of the bleaching composition containing thioureadioxide can be further enhanced by an addition of zinc sulphate.

The invention may conveniently; be carried out by bleaching the woodpulp in a bath which contains 0.2%- 110% of thiourea dioxide andapproximately five times this quantity of zinc sulphate, based on theweight of dry wood substance in the pulp to be bleached.

Optical bleaching agents may if desired be incorporated in the bleachingcomposition.

Sequestering agents such as poly'phosphates and ethylene diaminetetra-acetic acid may also be incorporated in the bleaching composition.

With some paper pulps, the process of bleaching with thiourea dioxide,in the presence or absence of zinc sulphate, proceeds with greaterefiiciency 'at increased pH values depending upon the source of thepulp. Suitable adjustments of pH can be achieved by the addition ofsodium silicate.

.The process of bleaching wood ,,pulp or similar fibrous material withthiourea dioxide may be combined with other'treatments designed toimpart specific properties to the finished product. The reduction offlammability of ICC paper boards by treatment with solutions of boratesor phosphates is well known; unfortunately, during the process theborate or phosphate solutions become coloured by organic materials inthe wood pulp and the treated material becomes discoloured.Discolouration can be greatly reduced, if the fire retarding treatmentis carried out simultaneously with a bleaching process with thioureadioxide. 'Qptimum bleaching efiects are obtained by using 0.05-3.00%weight/volume of thiourea dioxide in the flame retarding treatmentsolution.

The following are examples illustrating the method of the invention:

EXAMPLE 1 Ground wood pulp containing 20 g. dry wood substance weretreated in 100 cc. of a bleaching bath containing in solution 0.04 g. ofthiourea dioxide and 0.2 g. of zinc sulphate. The initial temperature ofthe bath was 40 C., and the temperature was raised within 20 minutes tothe boil, at which it was maintained for a further 20 minutes. Afterrinsing a bleached wood pulp with superior brightness was obtained.

' EXAMPLE 2 Ground wood pulp (1,000 g. containing 200 g. dry woodsubstance) was mixed with rapid stirring with 4,000 ccs. watercontaining1 g. pent-sodium tripolyphosphate (sequestering agent) and sodiumsilicate solution sufi'icient to bring the pH of the bath to 6.3-6.5;the temperature of the bath was raised. to C. and 0.5 g. thioureadioxide and 2 g. zinc sulphate (ZnSO -7H O) added. Bleaching was carriedout at 70 C. for one hour. After rinsing a bleached pulp was obtainedshowing a bright ness increment of 5 points.

EXAMPLE 3 was drawn. 0n drying the board showed considerable surfacediscolouration. The second pad was treated with a 14% solution of sodiumpolyborate containing in addition 0.25% i(w./v.) thiourea dioxide. Ondrying considerably less discolouration occurred than was the case withno thiourea dioxide. The third pad was treated with v a 14% polyboratesolution containing 1% thiourea dioxide;

the beneficial effects were even greater, and the board produced waseven paler than the fourth pad which was not treated with a fireretardant.

What is claimed is:

1. A process for bleaching wood pulp, which comprises bleaching the woodpulp in a composition containing from 0.2% to 1.0% of thiourea dioxideand 1% to-5% of zinc sulphate based on the weight of dry wood subtancein the pulp to be bleached.

2. A process as claimed in claim 1, in which the bleaching compositioncontains, in addition, a sequestering agent.

3. A process as claimed in claim 1, in which the bleaching is carriedout simultaneously with a fire retarding treatment using a solutioncontaining a fire retarding agent selected from the group consisting ofborates and phosphates.

References Cited UNITED STATES PATENTS 5 10/1941 Morgan 252-186 X10/1944 Loughborough 162-72 X 1/1949 Zcehuisen 162-72 X 6/1958 Sprout162-78 X 11/1958 Hawkinson et a1. 162-78 10 2,872,482 2/1959 Provoost260552 v 2,956,898 10/1960 Fleck 8-1 X 354,477 12/1886 Just et a1 162-79X 3,384,534 5/1968 Kindron et a1. 162-71 S. LEON BASHORE, PrimaryExaminer US. Cl. X.R.

